This invention relates to the filtering of signals in a closed loop servo system. There are in general no particular problems with filtering in closed loop systems where the input signal is available and hence measurable. But problems arise where only the error signal and the output signal are measurable. This occurs, for example, in a missile guidance loop where the summation of the target maneuver, constituting an input, and the missile response, constituting the output, occurs in the missle-target geometry. There only the error signal and the output signal are measurable. To reduce the undesirable effects of noise and other unwanted signals on the error signal, a filter can be inserted in the error signal path, but such a filter will affect the closed loop dynamics of the overall system. The ability to filter out undesired portions of the error signal is thus limited by the impact on the closed loop system.
Homing missiles developed for military applications have utilized proportional navigation guidance systems. They measure the rate of rotation of the line of sight between the missile and the target and command the missile to maneuver so that the rate of change of the line of sight is driven to zero. Typically in such missiles a space stabilized seeker or detector is mounted on a gimbal assembly beneath a radome. The radome is not ideally transparent in all directions to incoming signals such as RF or infrared signals. Missile wobble can cause the incoming signals to strike different parts of the radome causing incoming signal modulation. This may be erroneously interpreted as target motion and oscillation in the missile guidance loop may result, causing the missile to miss the target. Furthermore, noise attributable to stray incoming signals bouncing off of other objects and to sources in the electronic guidance circuitry can also introduce unwanted components into the error signal of the missile guidance loop. Clearly, missile accuracy can be improved by reducing or eliminating the extraneous signal components in the error signal due to radome loop coupling and noise sensitivity. However, since the measured line of sight rate also "contains" the missile dynamics, it is difficult to filter unwanted components of the error signal without interfering with the overall guidance loop.
Representative of filtered feedback control systems are U.S. Pat. Nos. 2,931,901; 3,184,662; 3,351,829; 3,470,430 and 3,808,486. Also of interest in this general field are U.S. Pat. Nos. 2,944,767; 2,965,823; 3,077,553; 3,084,320; 3,239,733; 3,246,221; 3,270,344; 3,409,251; 3,519,906; and 3,725,764.